The present invention relates to conductor compositions prepared in the form of paste or ink used to form film conductors on a ceramic base material such as a ceramic substrate. The present invention also relates to multilayer ceramic capacitors and other ceramic electronic components produced with the conductor compositions and a method for producing the same.
In compact and multifunctional electronic products such as mobile telephones, a large number of compact and precise ceramic electronic components such as hybrid ICs, multilayer capacitors, and multichip modules are used. FIG. 9 shows a schematic diagram of production procedure of a so-called multilayer circuit board 10 (e.g., low temperature sintered chip antenna switch module used for mobile telephones or the like) as a typical example of such highly functional ceramic electronic components.
First, a paste- or ink-like conductor composition (hereinafter, the compositions in these forms are collectively referred to as xe2x80x9cconductor pastexe2x80x9d) is applied to a sheet-like ceramic substance (green sheet) 11a prepared with a doctor blade or the like, and a film conductor 12e with a predetermined pattern made of the conducting component of the composition is formed ((A) in FIG. 9).
Then, a plurality of green sheets 11a, 11b, 11c, 11d, and 11e (including those in which film conductors 12b, 12c, 12d, and 12e having various patterns are previously formed) are pressed while being superposed in such a manner that the film conductors 12b, 12c, 12d, and 12e are buried inside ((B) and (C) in FIG. 9). Thus, a multilayer ceramic base material II provided the film conductors having a predetermined pattern as inner layers (hereinafter referred to as xe2x80x9cinner film conductors) can be obtained.
Then, the conductor paste is applied onto the surface (outer surface) of the multilayer ceramic base material 11, so that a film conductor 13 with a predetermined pattern made of the composition is formed (hereinafter referred to as a xe2x80x9csurface film conductorxe2x80x9d). Thereafter, the multilayer ceramic base material 11 in which the surface film conductor and the inner film conductors are formed is fired at a predetermined temperature, and thus the surface film conductor 13 and the inner film conductors 12b, 12c, 12d, and 12e are printed onto the ceramic base material 11 ((D) of FIG. 9).
After firing, the conductor paste is applied onto the side face (any one of the faces (end faces) adjacent to the surface on which the surface film conductor is formed, which also applies to the following) of the ceramic base material 11, and a film conductor 14 (hereinafter, referred to as xe2x80x9cside film conductorxe2x80x9d) having a predetermined pattern is formed. Thereafter, the multilayer ceramic base material 11 is heated so that the side film conductor 14 is fired for printing ((E) of FIG. 9). In general, nickel (Ni) plating for the purpose of preventing so-called solder leaching (solder penetration) and tin (Sn) plating for the purpose of providing solder wettability (adherence with solder) are performed with respect to the fired and attached surface film conductor 13 and side film conductor 14 ((F) in FIG. 9).
Then, predetermined elements 15 are mounted on the surface of the plated multilayer ceramic base material 11. This series of processes provides a multilayer circuit board (chip module) 10 ((G) in FIG. 9). Thus, the circuit board as well as other electronic components 2, 3, and 4 are mounted in a mother board 5 and thus a further higher order electronic product 1 is assembled ((H) in FIG. 9).
The conductor composition containing silver (Ag) or an Ag-based alloy in the form of a conductor metal powder (which may be referred to simply as xe2x80x9cAg pastexe2x80x9d in the following) is widely used as the conductor paste to form the film conductor in the ceramic electronic component such as the circuit board (chip module) 10. Ag is less expensive than gold (Au), platinum (Pt), palladium (Pd) or the like, and has a lower electrical resistivity than those metals, so that it is advantageous to use Ag to form film conductors in various electronic components. For example, Japanese Laid-Open Patent Publication No. 8-181514 discloses a method for producing a ceramic electronic component for high-frequency applications characterized in that the Ag paste is used to form the inner film conductors (internal electrodes) and the surface film conductor (external electrode).
In recent years, Ag pastes having better properties than those of a conventional one have been under in-depth development. For example, Japanese Laid-Open Patent Publication No. 11-163487 (Japanese Patent No. 3150932) discloses an Ag paste that can form a film conductor having only a small extent of bending even if it is fired together with ceramic green sheets. Japanese Laid-Open Patent Publication No. 2001-23438 discloses an Ag paste having an improved bond strength with respect to the ceramic base material. Japanese Laid-Open Patent Publication No. 11-111052 discloses an Ag paste having good solder wettability. Japanese Laid-Open Patent Publication No. 2000-265202 discloses a conductor paste that can form a film conductor having a small heat shrinkage ratio during firing and a method for preparing an Ag powder that is the main component of the paste. Japanese Laid-Open Patent Publication No. 9-194668 discloses an Ag paste that can suppress occurrence or progress of solder leaching (typically, dissolution of Ag contained in the film conductor into a solder), that is, has a high resistance to soldering heat. Japanese Laid-Open Patent Publication No. 8-7644 discloses an Ag paste that can form a dense film conductor that is hardly detached from the ceramic substrate.
In the Ag pastes disclosed in the above-described publications, some of general properties that are desirable for the Ag paste to form a film conductor are improved, but they are not developed with individual and specific focus on each of the surface film conductor and the side film conductor of the ceramic electronic component shown in FIG. 9. Therefore, when a conventional Ag paste was used as it was without any attempt for improvement, it was difficult to achieve both of the following to a high extent: quality improvement required especially for the surface film conductor fired at the same time as the ceramic base material (e.g., improvement of the bond strength or prevention of excessive firing shrinkage stress); and quality improvement required especially for the side film conductor that is applied after the firing and then fired for printing (e.g., suppression of solder leaching or improvement of resistance to soldering heat). Furthermore, the Ag paste serving as one base was not used with sufficient consideration on whether it should be used to form a surface film conductor or a side film conductor by changing the composition as appropriate (including a change of the content ratio of a metal powder of the main component or selection of a substance to be added as a secondary component).
It is an object of the present invention to provide Ag pastes that can realize the improvement of the quality required for each of two types of film conductor (i.e., surface film conductor and side film conductor) that are applied onto a ceramic base material at different times and fired at different times, and a method for using the Ag pastes to realize such quality improvement (including selecting an appropriate paste of the pastes having different compositions). It is another object of the present invention to provide a ceramic electronic component having improved electrical characteristics and/or mechanical characteristics (typically, a multilayer ceramic capacitor (MLCC) and other multilayer ceramic circuit boards) and a method for producing the electronic component.
The present invention provides improved paste-like or ink-like Ag based conductor compositions (Ag pastes), and provides the following method for producing a ceramic electronic component using the same.
A production method provided by the present invention is a method for producing a ceramic electronic component comprising a ceramic base material, a surface film conductor (typically thick film conductor) formed on a surface of the base material, and a side film conductor (typically thick film conductor) formed on a surface adjacent to the surface. The production method of the present invention is characterized by forming the surface film conductor and the side film conductor with a conductor composition (paste or ink) containing a metal powder substantially constituted by Ag or an Ag based alloy (referred to as xe2x80x9cAg based metal powderxe2x80x9d) and an organic medium in which the metal powder is dispersed. In the conductor composition (Ag paste), the surface of the Ag based metal powder is coated with an organic metal compound having as a constituent element any one selected from the group consisting of aluminum (Al), zirconium (Zr), titanium (Ti), yttrium (Y), calcium (Ca), magnesium (Mg) and zinc (Zn) (this means that organic compounds containing various metals can be used and the presence or the absence of carbon-metal bonds does not matter, which applies to the following) or an oxide of the metal.
In the specification of the present application, a xe2x80x9cceramic electronic componentxe2x80x9d refers to an electronic component in general that has a ceramic base material, and typically, a laminate (multilayered substrate) of several ceramic circuit boards (wiring substrates). Therefore, hybrid ICs, multichip modules and ceramic circuit boards constituting them or ceramic capacitors (typically multilayer ceramic capacitors) or the like are typical examples encompassed in the xe2x80x9cceramic electronic componentxe2x80x9d defined in the present specification.
According to this production method, improvement of the quality required for these two types of film conductor (including conductor layers that can be called electrodes or wiring from the functional point of view, which applies to the following), for example, improvement of the bond strength with the ceramic base material, improvement of the resistance to soldering heat, and solder wettability or prevention of excessive firing shrinkage during firing (which matters, in particular, for formation of the surface film conductor, which is fired together with ceramic green materials) can be achieved.
In carrying out the production method of the present invention, it is preferable to use an Ag paste in which the coating amount of the organic metal compound or an oxide of the metal is an amount corresponding to 0.01 to 2.0 wt % of the Ag based metal powder in terms of the metal oxide (i.e., the weight in terms of the metal oxide (e.g., Al2O3 or ZrO2) obtained when the organic metal compound is fired), and it is preferable to use an Ag paste in which the average particle size of the Ag based metal powder is 2.0 xcexcm or less. It is particularly preferable to use an Ag paste in which the Ag based metal powder is coated with an organic acid metal salt, metal alkoxide or a chelate compound having as a constituent element any one selected from the group consisting of Al, Zr, Ti, Y, Ca, Mg and Zn as the organic metal compound, or an Ag paste in which the Ag based metal powder is coated with a metal oxide obtained by heating such an organic metal compound.
In one preferable embodiment of the present invention, the two film conductors are formed with two respective conductor compositions (Ag pastes) having different compositions. More specifically, the production method includes (a) forming the surface film conductor with a first conductor composition on a ceramic base material, and (b) forming the side film conductor with a second conductor composition on the ceramic base material. The first conductor composition and the second conductor composition contain Ag based metal powder substantially constituted by Ag or an Ag based alloy as the main component, and the surface of the Ag based metal powder is coated with an organic metal compound or a metal oxide having as a constituent metal element any one selected from the group consisting of Al, Zr, Ti, Y, Ca, Mg and Zn. Thus, the second conductor composition has at least one of the features (1) the coating amount of the organic metal compound or the metal oxide is smaller than that of the first conductor composition and/or (2) at least one inorganic oxide powder that is not contained in the first conductor composition is contained as a secondary component, or the content ratio of the inorganic oxide powder contained also in the first conductor composition is high.
In the production method, the two Ag pastes whose differences in the composition are (1) and (2) as described above are used appropriately either for surface film conductor formation (first conductor composition) or for side film conductor formation (second conductor composition). Thus, improvement of the quality required for the surface film conductor (e.g., to prevent excessive shrinkage during firing so as to improve the bond strength with the ceramic base material) and improvement of the quality required for the side film conductor (e.g., to suppress soldering leaching without a plating treatment and improve the resistance to soldering heat) both can be achieved to a high level.
In particular, it is preferable that an oxide constituting the inorganic oxide powder is at least one selected from the group consisting of copper oxide, lead oxide, bismuth oxide, manganese oxide, cobalt oxide, magnesium oxide, tantalum oxide, niobium oxide and tungsten oxide. The lead oxide can be optionally added as the inorganic oxide powder, but it is not essential component for producing the Ag paste. Therefore, the production method of the present invention typically can provide a lead-free (Pb less) ceramic electronic component.
It is particularly preferable that the first conductor composition substantially does not contain the inorganic oxide powder that is contained in the second conductor composition (i.e., the first conductor composition and the second conductor composition are different in whether or not that inorganic oxide powder is contained) to achieve quality improvement of the two film conductors to a high level.
It is preferable that the content ratio of the Ag based metal powder in the second conductor composition is lower than that of the Ag based metal powder in the first conductor composition. Furthermore, it is preferable that the average particle size of the Ag based metal powder contained in the second conductor composition is smaller than that of the Ag based metal powder contained in the first conductor composition.
The surface film conductor and the side film conductor are formed in the following order. It is preferable that as shown in FIG. 9, first, in the process (a), the process of attaching the first conductor composition (paste) on an unfired ceramic base material and the process of firing the ceramic base material on which the composition is attached are performed (cofiring), and then in the process (b), the process of attaching the second conductor composition on the side face (end face) of the ceramic base material after the firing process is performed and the process of firing the ceramic base material on which the composition is attached are performed. When the film conductors are formed in this order, by utilizing the Ag pastes of the present invention, the quality required for each of the surface film conductor and the side film conductor can be improved to a high level.
According to another aspect of the present invention, a ceramic electronic component that can produced by the production method of the present invention is provided.
A ceramic electronic component provided by the present invention includes a surface film conductor and a side film conductor comprising a metal substantially constituted by Ag or an Ag based alloy, and a metal oxide having as a constituent metal element any one selected from the group consisting of Al, Zr, Ti, Y. Ca, Mg and Zn for coating the metal. The side film conductor is characterized in that (1) the content ratio of the metal oxide is smaller than that of the surface film conductor and/or (2) at least one inorganic oxide that is different from the metal oxide used for the coating and is not contained in the surface film conductor is contained, or the content ratio of such an inorganic oxide also contained in the surface film conductor is high.
As such an inorganic oxide, at least one selected from the group consisting of copper oxide, lead oxide, bismuth oxide, manganese oxide, cobalt oxide, magnesium oxide, tantalum oxide, niobium oxide and tungsten oxide can be used.
According to another aspect of the present invention, the conductor composition (Ag paste) that can be used in the method for producing a ceramic electronic component of the present invention can be provided. For example, the Ag paste for surface film conductor formation and the Ag paste for side film conductor formation having different compositions from each other can be provided.